Scientific Summary: The purpose of this project is to identify the critical regulators of remyelination in multiple sclerosis (MS) brains. It has been long established that oligodendrocyte progenitor cells (OPCs) populate MS lesions but do not transform into mature oligodendrocytes (OL) for reasons that remain unresolved. Therefore, one of the critical questions to ask is can we identify factors that prevent these cells from being able to mature and remyelinate? In recent years, microRNAs (miRNAs) have been identified as critical regulators of gene expression. We performed a comprehensive analysis of MS white matter lesions and identified 9 miRNAs that target 12 genes associated with the maturation of OPCs into mature OLs. Aim 1 in the current proposal will therefore validate these miRNAs and target genes in MS brains, specifically in grey matter. Aim 2 will build on these findings and provide in vitro evidence of the capacity of these miRNAs to induce myelination in OPCs. The next question to be answered is does loss of miRNAs lead to decreased remyelination by halting the progression of OPCs maturing into myelinating OLs in vivo? We propose to answer this question by using mice with loss of the miRNA processing enzyme Dicer in mature OLs and in OPCs. The role of miRNAs in promoting remyelination in vivo will be tested in Aim 3. The future of MS therapeutics lies in the identification of additional therapeutic targets and in developing combinatorial strategies. By characterizing failed myelin repair in MS brains, the studies outlined in this proposal should identify novel targets that will enhance repair of the MS brain.